Fundamental research for rare gas MRI

为进行核磁共振研究，研制了用于~3He核极化的圆柱形蓝宝石单晶盒。该池充满了4大气压的3He气体、少量的Rb液滴和100Torr的氮气。单晶的c轴平行于圆柱体的圆盘。该圆盘起到了激光的窗口作用，并对Rb原子的偏振进行了光学泵浦。蓝宝石单晶具有双折射特性。折射率取决于波振荡的方向。通过双折射窗口时的相移取决于振荡方向。由于相移，入射激光的圆偏振被退偏。相同尺寸的蓝宝石板补偿了相移。当原子碰撞时，通过超精细相互作用，Rb的极化转变为3He核的极化。用核磁共振方法监测了~3He的核极化。在~3He核偏振建立后，关闭激光器以观察~3He的偏振弛豫。~3He极化的驰豫时间为40h。对于玻璃电池，我们通常从10个电池中得到一个这样的电池。结果表明，蓝宝石是实现~3He偏振的良好材料。这是首次在蓝宝石电池中进行~3He极化测试。

A cylindrical sapphire single crystal cell has been developed for 3He nuclear polarization for a MRI study. The cell was filled with 4-atm 3He gas, a small amount of rubidium droplet and 100 Torr nitrogen gas. The c axis of the single crystal was parallel to the disk of the cylinder. The disk worked as a widow for a laser beam for and optical pumping of rubidium atomic polarization. The sapphire single crystal has birefringence. The refractive index depends on the direction of the wave oscillation. The phase shift upon passage through the birefringence window depends on the oscillation direction. The circular polarization of incident laser is depolarized because of the phase shift. A same size of sapphire plate compensated for the phase shift. The rubidium polarization is transferred to 3He nuclear polarization upon atomic collision via a hyperfine interaction. The 3He nuclear polarization was monitored by a NMR detection. After the buid-up of the 3He nuclear polarization, the laser was switched off for an observation of 3He polarization relaxation. The relaxation time of the 3He polarization was 40 h. In the case of glass cell, we usually obtained one such cell from 10 cells. The result shows the sapphire is good material for 3He polarization. This is the first test of 3He polarization in sapphire cell